Sensors manufactured by MEMS (micro-electro-mechanical systems) technology are used in, e.g., microphones and so forth. A microphone manufactured by MEMS technology is disclosed in the related art. This microphone includes a back plate as a fixed electrode, a diaphragm as a movable electrode, and a unit for keeping the diaphragm held to the back plate. The back plate has a plurality of holes. In the related art, there is known a structure in which the back plate united with a silicon wafer is formed by etching the silicon wafer. A cavity is formed at the rear surface side of the silicon wafer with respect to the back plate. A spacing is formed at the front surface side of the silicon wafer with respect to the back plate. A diaphragm made of silicon nitride is arranged on the front surface of the silicon wafer to face the back plate across the spacing. A metal layer is formed on the front surface of the diaphragm. If a sound wave is inputted, the diaphragm vibrates to thereby change the capacitance between the metal layer on the front surface of the diaphragm and the back plate. Thus, the sound wave can be converted to an electric signal.
A method for forming the diaphragm includes a step of forming a polysilicon film as a sacrificing layer on the front surface of the silicon wafer, a step of forming a silicon nitride film (diaphragm) on the polysilicon film, and a step of etching and removing the polysilicon film as the sacrificing layer to release restraint of the diaphragm.
In order to enhance the sensitivity of a capacitance type sensor, it is desirable to increase the area (the mutually-facing area) of a movable electrode and a fixed electrode. In the related art, however, the device size grows larger as the area of the electrodes is increased. This leads to a decrease in the number of sensors, e.g., microphones, which can be manufactured from a single silicon wafer. Accordingly, the manufacturing cost of sensors increases.
In the related art, it is unavoidable to form a polysilicon layer as a sacrificing layer in order to support the diaphragm in a movable state. While the polysilicon layer is conventionally removed through the use of a “wet etching agent,” the present inventor has found through inspection that, in the wet etching process, the diaphragm adheres to the surface of the silicon wafer by the surface tension of an etching liquid. Therefore, in reality, the removal of the polysilicon layer as a sacrificing layer needs to be performed by gas-phase etching through the use of hydrofluoric acid vapor. However, gas-phase etching is a costly process, becoming an obstacle in reducing the manufacturing cost.